The invention relates to a method of providing mask alignment marks on an active surface of a semiconductor substrate, in which first at least one layer of a material resistant to oxidation is formed on said active surface, after which by a local etching of this layer, zones for isolation by a thick oxide, designated as field oxide, are defined simultaneously with the alignment marks, and during a later step the zones for the isolation by field oxide and the said alignment marks thus defined are subjected to a thermal oxidation, the remaining part of the active surface being protected from the oxidation by a remaining part of the layer anti-oxidation material.
A prior art method of this kind is known from the document JP Abstract 61-100928, 157 E 439.
According to the known method, the alignment marks are defined simultaneously with the isolation zones by field oxide. Thus, a relative positioning of these elements is obtained which corresponds to the accuracy of the apparatus used and is therefore free from a possible realignment error at this stage.
In the manufacture of a microcircuit of the type comprising MOS transistors, it is in fact the first operation to define a structure located at the surface of the substrate. The later masking operations necessitate an alignment of the masks as precise as possible by reference to the preformed realignment marks. With the advent of the submicron technologies, it becomes necessary to reduce as far as possible the positioning errors of the masks with respect to the alignment marks defined on the substrate and for this purpose use is made of very elaborate optical methods, which provide an automatic alignment with a tolerance of 0.2 .mu.m by means of servo-mechanisms of high precision.
However, such mask alignment methods require necessarily that the alignment marks are presented in optimum geometric form and depression depth with a view to their use in an optical method based on the formation of interference figures.
The known method is based on the creation of depressions by an etching of the alignment marks utilizing as a mask, field oxide parts reserved to this end.
This results in that the alignment marks are then composed of depressed parts, in which the substrate is exposed, and other parts, which are covered by field oxide, which leads to difficulties for the formation of the interference figures having a high contrast. On the contrary, it would be desirable to obtain alignment marks protected by an oxide layer preferably having a uniform thickness while maintaining a minimum number of operating steps.